kernel: Correctly deal with invalid base16 strings in ";;guix download".

Previously we'd end up with an uncaught exception.

* guix-jupyter-kernel.scm (handle-download): Wrap
'base16-string->bytevector' in 'false-if-exception'.

guix-jupyter-kernel.scm

@@ -329,7 +329,7 @@ HASH of type HASH-ALGO."
     (lookup-hash-algorithm (string->symbol hash-algo)))
 
   (define hash/bv
-    (base16-string->bytevector hash))
+    (false-if-exception (base16-string->bytevector hash)))
 
   (define count
     (proxy-state-execution-count state))